The present invention is related to hydrogen sensors, and more particularly, to a protective coating for a single-chip hydrogen sensor.
During the early 1990s, Sandia National Laboratory developed a single-chip hydrogen sensor that utilized Palladium-Nickel (PdNi) metal films as hydrogen gas sensors. U.S. Pat. No. 5,279,795. naming Robert C. Hughes and W. Kent Schubert as inventors, assigned to the United States as represented by the U.S. Department of Energy, describes such a sensor and is incorporated by reference herein. One of the key benefits of the sensor described in the ""795 patent is its ability to detect a dynamic range of hydrogen concentrations over at least six orders of magnitude. Prior solutions to the problem of detecting hydrogen concentrations had been generally limited to detecting low concentrations of hydrogen. These solutions include such technologies as metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field-effect-transistors (FET), as well as palladium-gated diodes.
The hydrogen sensor described in the ""795 patent was a notable advance in hydroponic-sensing technology. It was, however, primarily limited to an experimental laboratory environment due to the difficulties encountered in manufacturing such a sensor. Difficulties in producing such semiconductor devices due to the specialized materials used might result in low device production yields. An economically feasible commercial hydrogen sensor is difficult to obtain if yields are under an acceptable level.
The assignee of the present invention has developed techniques to improve device yields in an attempt to manufacture a commercializable single-chip hydrogen sensor. Two of these techniques are described respectively in U.S. patent application Ser. No. 09/729,147, titled xe2x80x9cRobust Single-Chip Hydrogen Sensor,xe2x80x9d filed Dec. 1, 2000, and U.S. patent application Ser. No. 09/878,668, titled xe2x80x9cManufacturable Single-Chip Hydrogen Sensor,xe2x80x9d filed concurrently herewith, both of which are incorporated by reference herein.
These improved sensors are still, however, prone to possible yield problems. A major source of yield problems arises from the poor adhesion of the hydrogen sensing material (typically a PdNi thin film) to the underlying chip structure. As a result, although the PdNi films may survive processing and initial testing, many of the PdNi structures are lost during subsequent wafer dicing. Wafer dicing is practically a requirement to producing a commercializable hydrogen sensor, due to the efficiencies that arise from mass producing sensors on a single wafer.
Initial attempts have involved depositing a protective coating, such as photoresist, over the wafer, prior to dicing. Although this technique shows some promise in preventing the loss of PdNi, the yield has not substantially improved. Moreover, the protective layer must be removed before the chip can sense hydrogen. Such a removal operation can lead to further yield losses.
In addition to protecting sensing elements during wafer dicing, there is also a need to provide protection from hazards that might be encountered in the sensing environment. For example, a durable coating might be desired to protect the sensing elements from physical damage, such as scratches. Also, the particular testing environment might contain chemicals that might be harmful to an unprotected sensor chip. For example, an environment that contains hydrochloric acid might damage the sensing elements on an unprotected sensor.
To protect against environmental hazards, individual sensors have recently been covered With a permeable membrane that allows a gas to pass through while providing some degree of protection for the sensor. See, for example, Stuart, M., xe2x80x9cUsing Hydrophobic Membranes to Protect Gas Sensors,xe2x80x9d Sensors, p. 14, May 1998. Such a technique, however, does not protect the on-chip components during wafer dicing and/or assembly, because the membrane is applied after processing and wafer dicing.
Thus, it would be desirable to provide a protective coating for a single-chip hydrogen sensor that is capable of providing protection during the manufacture of the sensor, and that allows hydrogen levels to be sensed while the protective coating is in place.
It would also be desirable to provide a robust single-chip hydrogen sensor that is capable of sensing hydrogen concentrations over a broad range, such as from approximately 1% to approximately 100% concentrations.
It would also be desirable for such a sensor to be efficiently manufacturable, so that costs are reduced and the sensor is producible in high enough yields to enable commercialization.
It would be desirable for such a sensor to provide measurement results that approximate or improve on the results from previous hydrogen sensors.
It would additionally be desirable to minimize sensor drift and to improve device-to-device and wafer-to-wafer repeatability.
In accordance with an illustrative embodiment of the present invention, some of the problems associated with manufacturing a robust hydrogen sensor are addressed.
In a first aspect of the invention, a semiconductor wafer including a plurality of hydrogen sensors is provided. The wafer includes a hydrogen-permeable coating disposed over the plurality of hydrogen sensors. The plurality of hydrogen sensors may each include one or more sensing elements, such as palladium nickel sense resistors or transistors. In a further embodiment, the semiconductor wafer further includes at least one temperature sensor for determining a sensor temperature and at least one temperature controller for controlling the temperature of the sensor. The hydrogen-permeable coating is preferably an organic spin-on polymer.
In a second aspect of the invention, a robust hydrogen sensor is provided. The sensor includes a silicon base portion, at least one hydrogen-sensing element disposed on the silicon base portion, and a hydrogen-permeable coating disposed over the at least one hydrogen-sensing element. The sensor may, for example, include at least one palladium nickel sense resistor for determining hydrogen concentration within a first range and at least one palladium nickel sense transistor for determining hydrogen concentration within a second range. The hydrogen-permeable coating is preferably an organic spin-on polymer.
In a third aspect, a method for fabricating a robust single-chip hydrogen sensor is provided. The method includes providing a plurality of hydrogen sensors on a wafer, applying a hydrogen-permeable coating to the wafer, and separating the wafer into a plurality of dies. The method may further include providing a temperature sensor and a temperature controller on the wafer. The hydrogen-permeable coating may, for example, be an organic polymer.
In a fourth aspect, a method for fabricating a robust single-chip hydrogen sensor is provided. The method includes applying a hydrogen-permeable coating over at least one hydrogen-sensing element disposed on a wafer. For example, the hydrogen-sensing element may include at least one palladium nickel sense resistor for determining hydrogen concentration within a first range and at least one palladium nickel sense transistor for determining hydrogen concentration within a second range. The hydrogen-permeable coating is preferably an organic spin-on polymer, which may be applied by securing the wafer on a chuck, rotating the chuck, thereby rotating the wafer, and dripping, the hydrogen-permeable coating onto the wafer. Other application processes, such as spray-coating, dip-coating, physical deposition, chemical vapor deposition, evaporation, and sputtering, may also be used.